The present disclosure relates to a fixing device which is employed in a copying machine, printer, facsimile machine, multifunctional peripheral and the like, and an image forming apparatus having the same. The present disclosure particularly relates to a fixing device in which a pressing member is used to press a fixing belt against a pressure applying roller and an image forming apparatus having this fixing device.
An image has hitherto been recorded on an image recording medium in such a manner: A toner image formed on an image carrier such as a photosensitive drum is transferred to an image recording medium. This image recording medium carrying the toner image is conveyed to a fixing device where heat and pressure are applied to the toner image such that the toner image is fixed to the image recording medium. A type of fixing device using a belt for fixing a toner image may be named as an example. This type of device employs an endless fixing belt which is heated and with which a pressure applying roller comes in pressure contact to form a fixing nip. When the image recording medium carrying an unfixed toner image passes through the fixing nip, it is fixed to the recording medium.
In this type of fixing device using a fixing belt, a pressing member is disposed on a side of an inner surface of the fixing belt and presses the fixing belt against the pressure applying roller, so that a fixing nip is formed between the fixing belt and the pressure applying roller. The rotationally driven pressure applying roller causes the inner surface of the fixing belt to slide relative to the pressing member, such that the fixing belt is driven to rotate. When the rotationally driven pressure applying roller drives the fixing belt to rotate, it has occurred that the fixing belt slips and does not smoothly rotate depending on an amount of frictional resistance between the pressure applying roller and the fixing belt. The slippage has occurred when an amount of frictional resistance between the pressure applying roller and the fixing belt or between the image recording medium and the fixing belt is smaller than an amount of sliding resistance between the fixing belt and the pressing member.
Techniques related to solving the problem of slippage of a fixing belt have hitherto been known. With respect to the above-mentioned type of fixing by a belt provided with the pressing member, a first example having an end cap and a gear is known. The end cap mates with an opening at an end of the endless fixing belt in a direction of a rotational shaft. The gear is configured to be integral with the end cap. This gear meshes with a drive gear, which meshes with a roller gear that drives the pressure applying roller to rotate. Rotation of the pressure applying roller causes the end cap to rotate via a gear train. It is so configured that the fixing belt rotates following rotation of the pressure applying roller and rotation of the end cap causes the fixing belt to rotate substantially at the same speed of the pressure applying roller. In this manner, the problem of slippage of the fixing belt is solved.
In addition, with respect to the type of fixing by a belt having a pressing member, a second example is known, in which a drive roller configured to drive a fixing belt to rotate is employed in addition to a pressure applying roller. In this example, two drive rollers are disposed coaxially with the pressure applying roller axially outside both ends of the pressure applying roller. The two drive rollers are in pressure contact with flange members while interposing the fixing belt therebetween, the flange members being configured to be rotatable outside a region of a fixing nip. The fixing belt rotates following not only rotation of the pressure applying roller rotationally driven by a drive source, but also rotation of the drive rollers. Since a rotational driving force applied to the fixing belt increases in this manner, it is possible to prevent the fixing belt from slipping.
In addition, with respect to the type of fixing by a belt having a pressing member, a third example is known, in which flange members are attached to both ends of an endless fixing belt in such a manner that the flange members are mated around an outer surface of the fixing belt. When a pressing member and a pressure applying roller comes in pressure contact with each other to form a fixing nip, the fixing nip causes the fixing belt to change its shape into an elliptic shape, so that an outer surface of the fixing belt comes in tight contact with inner surfaces of the flange members. Accordingly, rotational driving forces of the rotationally driven flange members are transmitted to the fixing belt. Since the rotational driving forces of the flange members, which are added to a rotational driving force of the pressure applying roller, assist rotation of the fixing belt, it is possible to prevent the fixing belt from slipping.
However, in the first example of the fixing device, the fixing belt has variations in a shape of deformation in an axial direction, since the fixing belt changes its shape at the fixing nip according to a shape of the fixing nip, whereas the fixing belt changes its shape into a circular shape at an axial end of the fixing belt. If the fixing belt repeats rotation under the condition described above, it may be that as a compressive stress and a tensile stress act on the fixing belt, the fixing belt suffers from stress destruction. On the other hand, if the end cap is arranged spaced much away from the fixing nip to decrease the stress due to extension and contraction of the fixing belt, it will lead to a problem of a dimensional increase of the device in a direction of a rotation shaft.
In the second example of a fixing device, it may be that costs increase due to more time required for processing the fixing belt, since a treatment is applied to the fixing belt such that surface roughness of the fixing belt differs between the fixing nip and a part in pressure contact with the drive roller. With applying the treatment, the amount of the frictional resistance is adapted to be larger for the part of the fixing belt in pressure contact with the drive roller
The third example of a fixing device, in which the outer surface of the fixing belt comes in tight contact with the inner surface of the flange member when the fixing belt changes its shape into an elliptic shape, has a problem that the rotational driving force is not sufficiently transmitted from the flange members to the fixing belt.